For many years petroleum companies concentrated on developing oil and gas fields on land. But the world's appetite for energy sources, coupled with diminishing returns from land drilling, has driven petroleum companies to develop offshore reserves.
Sub-sea geologic sediments and structures are often similar and in some cases superior to geologic conditions that have proven highly productive on land. In fact, offshore reserves have been estimated at 21% of the world's proven reserves, with estimates that 40% to 50% of all future resources will come from offshore reserves.
Drilling offshore wells in deep water, greater than about 600 feet of depth, creates its own set of problems. When drilling on the edge of the continental shelf, quite frequently, geo-pressured, water-bearing sands also known as shallow water flows (SWF) are encountered at about 1000 to 2000 feet below the mud line. The depth of these sands and the pressures that they exhibit create problems for well operators.
One of the problems faced by the operators is the nature of the sands that are associated with SWF. These sands are thought to be made primarily of quartz. The grains of sand are believed to be rounded, well sorted, and have an average grain diameter of 100 microns or more. These grains of sand are loosely packed and unconsolidated, similar to the sand found on beaches and on river sand bars, thereby contributing to the instability of the formation.
Perhaps the greatest challenge faced by the operators drilling offshore wells is to control the formation pressure throughout the drilling and cementing process. Typically drilling fluids are weighted to increase the density of the drilling fluid in order to inhibit the flow of water from the formation into the borehole. However, in areas of SWF there is only a narrow range of drilling fluid densities that can be used to control the formation pressure of the wellbore.
If the drilling fluid is too light, water will flow into the well and can result in the well washing out. Early on, operators thought it might be okay to let some of the water flow into the well to relieve pressure on the aquifer. Experience has proven, however, that once water begins flowing into a well, it is hard to regain control of the well, and, typically, a well ends up being washed out and lost.
On the other hand, if the drilling fluid is too heavy the pressure in the wellbore may exceed the fracture pressure in the sands leading to the fracturing of the formation. Once a fracture has been induced, the fracture will typically widen and may even grow into the next well in a template. In addition, these fractures can be reopened at lower circulating pressures during subsequent drilling prior to casing off the formation. As fractures are formed and widened more and more drilling fluid is lost to the formation, causing a massive loss of circulating drilling fluids.
If the annulus of the borehole does not remain full of drilling fluid, the hydrostatic pressure within the borehole may decrease until the formation fluids, previously controlled by the drilling fluid hydrostatic pressure, are allowed to flow into the borehole. The end result can be a kick, a blowout, or previously stable formations can collapse into the borehole. Borehole collapse results in severe wash-outs and borehole enlargement to the extent that it may not be possible to continue drilling.
Furthermore, if the drilling fluid is circulated to the drilling rig at the surface, the weight of the column of drilling fluid being circulated to the surface adds to the hydrostatic head of pressure at the wellbore and limits the weight and, consequently, the composition of the drilling fluid that can be used. Thus, two methods of drilling off shore wells have been developed. These two methods are called riserless drilling and drilling with a riser.
A riser is a piece of casing that connects the sub-sea well to the drilling rig and allows one to circulate drilling fluid to the surface. During riserless drilling, drilling fluid is used for a single pass through the wellbore and is then discharged directly to the sea floor. If one drills riserless in areas having SWF, drilling fluid weights of about 11 to 12 lbs/gal are required. The desirable drilling fluid weight depends on the environment of the well, including the water depth and the depth below the mud line of the SWF.
Controlling the density of the drilling fluid in riserless drilling is more difficult because the drilling fluid transports cuttings and drill solids and there is no way to determine how many of these there are in the drilling fluid at any one time. Furthermore, riserless drilling requires large volumes of drilling fluid. Often, riserless drilling requires more fluid than can be stored on a drilling rig and subjects the drilling process to interruptions due to bad weather and the disruption of supply boats delivering drilling fluid. The direct discharge of large volumes of cuttings and drilling fluids on the sea bed may raise environmental questions. For riserless drilling, operators use simple muds: this is both for reasons of cost and environmental concerns. Mixed metal silicate muds and calcium chloride brines are examples of muds suitable for riserless drilling of SWF.
Drilling with a riser provides more flexibility in selecting the optimum drilling fluid components. However, the additional hydrostatic pressure provided by the column of drilling fluid rising to the surface leads to the critical question: "How does one formulate a fully functional drilling fluid and remain below drilling fluid weights that propagate fractures?" Drilling fluid weights for drilling SWF with a riser should be in the range of about 8.5 to about 10 lbs/gal. Again, the desirable drilling fluid weight depends on water depth and depth below the sea-floor of the SWV. To date it has been difficult and sometimes impossible to formulate a fully functional drilling fluid while maintaining a drilling fluid of the required light weight.